A roll stabilizer is an apparatus on a motor vehicle body which increases the stiffness of a suspension system of the motor vehicle on one side or the other of the vehicle in response to pivotal movement of the body about a longitudinal roll axis to reduce the amplitude of such pivotal movement. A typical roll stabilizer is a U-shaped torsion bar on the vehicle body connected at its opposite ends to respective ones of a pair of suspension control arms such that the torsion bar is torsionally flexed by opposite suspension excursions of the control arms. The torsion bar, however, is also torsionally flexed by unequal excursions of the control arm in the same direction such as occur when the motor vehicle traverses an irregular road surface. Since an increase in the stiffness of the suspension system on an irregular road surface directly affects passenger comfort, torsional flexure of the torsion bar on such irregular road surfaces is usually a negative characteristic of typical roll stabilizers. A roll stabilizer described in U.S. Pat. No. 5,186,486, issued Feb. 16, 1993 and assigned to the assignee of this invention, includes an active link between an end of a U-shaped torsion bar and a suspension control arm. The rigidity of the active link is varied dynamically, i.e. during operation of the motor vehicle, by an electronic controller to selectively reduce the effective torsion modulus of the torsion bar when roll stabilization is not required. Similarly, U.S. Pat. No. 4,796,911 describes a roll stabilizer including a split U-shaped torsion bar having its opposite ends connected to respective ones of a pair of suspension control arms and a coupling filled with magnetic fluid between the two separate elements of the torsion bar. An electronic controller dynamically varies the stiffness of the coupling and, hence, the effective torsion modulus of the torsion bar by varying a magnetic field to which the magnetic fluid is exposed. A roll stabilizer according to this invention is a novel alternative to these dynamically variable roll stabilizers.